This invention relates to image display and more particularly to a method of image encoding by which pixel information for an image is encoded into digital data which can be recorded on a suitable storage medium. The invention also relates to a method of decoding by which such digital data is decoded to restitute the original pixel information. The invention further relates to arrangements for carrying out these encoding and decoding methods and to data display apparatus embodying such a decoding arrangement.
In a typical electronic image display apparatus, an image which is displayed on the screen of a CRT (cathode ray tube) or other display device is composed of discrete pixels each of which is represented by a respective digital code which defines a pixel in terms of chrominance and luminance pixel component values. Each of these digital codes is stored in a display memory at a location corresponding to the position of the pixel which it represents in the displayed image. The display memory is addressed periodically in synchronism with the operation of the display device to read out the digital codes which are used to produce video signals for driving the display device. The display produced by electronic image display apparatus of the above type is termed a bit-map display and has, for example, a resolution of 360.times.280 pixels. The apparatus can include a background memory in which is stored the pixel information for a large number of images. When a displayed image is to be replaced by a new image, the pixel information for the new image has to be made available in the display memory as the respective digital codes which represent the discrete pixel values for the new image. The pixel information can be stored in the background memory as the actual respective digital codes which represent the discrete pixel values, so that the digital codes for the new image can be read out from the background memory and written directly into the display memory in place of the digital codes for the previously displayed image.
The time taken for this read/write operation in respect of the digital codes for the new image depends inter alia upon the operating speed (i.e. the speed of data access) of the background memory. When the background memory is a mass memory, such as an optical record carrier (i.e. a compact disc) in its role as a read only memory (CD ROM), its operating speed may be too low for certain applications. In particular, it has been determined empirically that a user of image display apparatus of the above type will tolerate a delay of only about one second for replacing a displayed image by a new image. If the delay is significantly longer, then use of the apparatus becomes aesthetically unacceptable. As a consequence, the amount of pixel information that can be transferred from the background memory to the display memory to change the image being displayed becomes limited.
It has been found that such a limitation poses a problem in respect of enhanced resolution displays which require a large amount of pixel information. A CD ROM can readily provide the storage capacity which is necessary, but accessing this pixel information sufficiently quickly has proven to be difficult. As an example, for a normal resolution image display using a 360.times.280 pixel matrix, as aforesaid, the time taken to load digital codes representing discrete pixel values from a CD ROM into a display memory is thought to be on the margin of user acceptability. However, for an enhanced resolution image display using a 720.times.560 pixel matrix, (i.e. having a quadruple number of pixels), which is contemplated, the loading of digital codes representing discrete pixel values will take four times longer, which is unacceptable.